Purpose Soils that develop on the dumps in historical arsenic mining sites contain high concentrations of As thus constituting a serious environmental risk. This study was aimed to examine the changes in arsenic solubility in mine soils as induced by organic matter introduced with forest litter. Materials and methods Four large samples of initially developed soils were collected from the dumps remaining in former mining sites and were incubated for 90 days at various moistures: 80% of maximum water holding capacity and 100% (flooded conditions), with and without addition of beech forest litter (BL), 50 g/kg. Soils contained up to 5.0% As. Soil pore water was collected periodically with MacroRhizon suction samplers and examined on As, Mn, and Fe concentrations, pH, Eh, and dissolved organic carbon (DOC). The properties of dissolved organic matter were characterized by UV-VIS spectroscopic parameters A4/A6 and SUVA . Results and discussion ApplicationofBLresultedinanintensive release of As from soils, particularly at 100% moisture. As concentrations in soil pore water increased strongly during the first 2 or 4 weeks of incubation and then started to decrease in all cases, except for one flooded soil. As was released particularly intensively from carbonate-containing soils. The mechanisms of As mobilization, including reductive dissolution of Mn and Fe oxides andthe competitionwithDOC forsorptionsitesonthe oxides, were discussed as related to soil properties. Pore water concentrations of DOC were increasing at the beginning of incubation and started to decrease after two or four weeks. Spectroscopic parameters of dissolved organic matter in ZS soils indicated increasing aromaticity and prog- ress of humification. Conclusions Forest litter introduced to mine dump soils causes a mobilization of As into soil pore water. This effect, particularly strong in carbonate-rich soils, is apparently related to high concentrations of DOC and usually declines with time, which may be explained by the progress in humification. The relationships between DOC properties and As speciation in soil pore water should be dissected for better interpretation of experimental results. . . . . . . Keywords Arsenic Dump Dissolved organic carbon Incubation MacroRhizon Organic matter Pore water Responsible editor: Anja Miltner Electronic supplementary material The online version of this article (https://doi.org/10.1007/s11368-018-2031-2) contains supplementary material, which is available to authorized users. * Anna Karczewska Department of Soil Science and Remote Sensing of Soils, Adam firstname.lastname@example.org Mickiewicz University in Poznań, ul. Krygowskiego 10, 61-680 Poznań,Poland 1 Institute of Geology, Adam Mickiewicz University in Poznań,ul. Institute of Soil Science and Environmental Protection, Wroclaw Krygowskiego 12, 61-680 Poznań,Poland University of Environmental and Life Sciences, ul. Grunwaldzka 53, 50-357 Wrocław, Poland Department of Agroecosystems and Green Areas Management, Wroclaw University of Environmental and Life Sciences, pl. Grunwaldzki 24a, 50-350 Wrocław, Poland J Soils Sediments 1 Introduction poses on typical heavy metals. The main mechanisms in- volved in As release from soil in the presence of organic Arsenic is a toxic element; therefore, its mobility in the envi- matter include competition for available adsorption sites, for- ronment and lability in soil are a matter of concern (Wenzel mation of aqueous complexes, and changes in the redox po- 2013). Arsenic concentrations in Polish soils are basically tential (Wang and Mulligan 2006). Not many papers were very low, but there are sites associated with historical ore published that reported As bonding to solid or aqueous humic mining, where arsenic occurs in very high concentrations substances (Thanabalasingam and Pickering 1986;Silvettiet (Karczewska et al. 2007, 2013b; Krysiak and Karczewska al. 2017; Saada et al. 2003). 2007). Particularly, high amounts of As were recorded in mine Several studies carried out with forest soils concluded that dumps. The release of As from dump material may likely be As did not accumulate in forest litter and that the concentra- caused by weathering, leaching, and other processes. Arsenic tions of As in a humus layer are usually much lower than those species strongly bound to iron, aluminum, and manganese in underlying mineral horizons of As-enriched soils (Blaser et oxides form usually the main pool of As in soils (Bauer and al. 2000;Karczewska etal. 2013b). Some authors reported, Blodau 2006; Wenzel 2013). However, especially in soils however, accumulation of As in peat and in organic soils, as where total As concentration is high, a substantial amount of well as in the forest humus (González et al. 2006;Brunetal. As may remain as potentially soluble or exchangeable. Under 2008; Langner et al. 2012), but a thorough insight in the bal- certain conditions, however, arsenic distribution in soil can be ance of As in forest humus provides a divergent picture, relat- modified causing the release of its considerable amounts from ed strongly to climatic conditions. Generally, in the conditions soil solid phase into soil solution. Such an effect may be of high humidity, As indicated the tendency to being leached caused by the presence of anions, such as phosphates, that from forest litter (Brun et al. 2008). compete with arsenates for oxide sorption sites and also by Various factors and mechanisms are involved in the pro- the changes in soil pH or redox potential (Bolan et al. 2013; cesses of As mobilization from soils by organic matter, in Lewińska and Karczewska 2013; Wenzel 2013;Arco-Lázaro particular by dissolved organic matter (DOM), including sorp- et al. 2016). Important parameters that affect the processes of tion/desorption, sorption competition, redox reactions, com- As sorption and desorption are soil pH (Carbonell-Barrachina plexation, and colloid formation (Bauer and Blodau 2006; et al. 1999; Wenzel 2013; Komárek et al. 2013), as well as the Wang and Mulligan 2006; Moreno-Jiménez et al. 2013;Xie presence or absence of carbonates. Fakhreddine et al. (2015) et al. 2015). Various soil moisture conditions, with special 2+ 2+ stressed that Ca and Mg cations present in water at high case of waterlogging, are of crucial importance in terms of concentrations foster arsenate adsorption to the phyllosilicate As solubility, as they determine soil redox potential and gov- clay minerals, while at the absence of those cations in water ern the processes of organic matter transformation. The prod- ucts derived from plant residues decomposing under oxidized As gets easily released from the sediments into solution. However, Kim et al. (2000) and Han et al. (2007)proved that and anoxic conditions differ considerably if considering their the presence of carbonates in soil may cause enhanced release properties, including molecular size, aromaticity, and the of arsenic (As III) under anoxic conditions due to formation of kinds of functional groups (Łabaz et al. 2016; Berg 2017; stable arseno-carbonate complexes, believed to be As(CO ) , Kawałko et al. 2017). These factors are important for the run 3 2 − + As(CO )(OH) , and AsCO . of sorption/desorption processes or organic matter 3 2 3 Application of organic matter is another factor that may complexing properties. Humic substances together with non- affect the distribution of As in soils, thus changing its potential specific organic compounds make a complex mixture of prod- solubility. Various authors reported increased solubility of ucts that cannot be described by simple chemical formulas heavy metals and arsenic in soils caused by the presence of (Chen et al. 1977; Swift 1996). Therefore, various indices plant cover or induced by the products that are formed in soils are used to depict the concentrations and features of soil or- from decomposing plant residues. In particular, the processes ganic matter. The indices most commonly used to characterize of forest litter transformation may lead to increased solubility organic compounds dissolved in water are DOC concentra- of various elements, as they deliver organic and inorganic tions and optical properties based on spectral characteristics compounds into draining water, stimulate biological activity, of light absorption in the ranges of UV, VIS, and IR. Specific and additionally may affect redox conditions (Kalbitz and UV absorbance SUVA is defined as the UV absorbance of Wennrich 1998; Karczewska et al. 2013a, 2017; Kabałaet water sample at 254 nm, measured in inverse meters (1/m), al. 2014;Berg 2017; Cuske et al. 2017). related to the concentration of DOC (in mg/L). SUVA is an The effects of organic matter on the solubility of As in soils average absorptivity of all the organic molecules comprised depend obviously on its properties, but almost all the recent within the DOC in a water or soil solution sample. It may be studies proved that both dissolved and particulate or solid used as a surrogate index of DOC aromaticity (Weishaar et al. organic matter may cause an increased solubility of As, con- 2003). The A4:A6 parameter (often denoted as E4:E6) pro- trary to the effects that high molecular weight organic matter vides a simplified information on the humification degree of Odra Central Sudety Mts. Bóbr Eastern Sudety Mts Western Sudety Mts. J Soils Sediments organic matter (Chen et al. 1977; Strobel et al. 2001; Łabaz including waterlogging. Similar studies on mine soils were and Gałka 2012; Łabaz et al. 2016). only occasionally performed (Kalbitz and Wennrich 1998; Under anoxic conditions, the concentrations of DOC in the Karczewska et al. 2013a;Van Neveletal. 2013, 2014;Cuske soil solution underneath the forest floor are commonly elevat- et al. 2017) and their results indicated the risk of forest litter- ed, due to the absence of oxide sorption sites (caused by pod- induced release of metals and metalloids from polluted soils. zolization processes) and greatly reduced microbial activity. In such conditions, easily degradable organic compounds tend to concentrate in soil pore water (Marschner and Kalbitz 2003) 2 Materials and methods and compete with inorganic ions for soil sorption sites. Anoxic conditions are of particular importance for the release 2.1 Site description of As from soil solid phase due to reductive dissolution of FeOx (Komárek et al. 2013; Wenzel 2013) driven by the drop Soils at initial stage of pedogenesis, developing from weath- in redox potential (Eh). Several studies have shown, however, ered waste rock material of mine dumps, were collected in a that the As release from enriched soils depends on its specia- historical site of former Au and As mining in Złoty Stok (ZS1, tion and stability of As-sorbing oxides, which means that the ZS2) and in the sites of polymetallic ore mining in the villages most tightly bound As forms remain permanent and will not of Radzimowice (R) and Dziećmorowice (DM) (Fig. 1). be released even in waterlogging conditions (Weber et al. Those sites have different geological settings and ore miner- 2009; Krysiak and Karczewska 2011; Komárek et al. 2013; alogy and consequently differ in the properties of waste rocks. Martin et al. 2014). Additionally, the dynamics of water- Złoty Stok, formerly German Reichenstein, was known from soluble As in soils is controlled by the period of flooding environmental arsenicalism, i.e., the cases of arsenic caused (Shaheen et al. 2014). diseases (Kathe 1937; Hindmarsh et al. 1977). Ore minerali- This study was aimed to examine the release of As into soil zation in Złoty Stok is related to metamorphogenic, hydrother- solution from mine dump soils differing in properties, as af- mal As-Au deposits that formed in a dislocation zone. fected by introduction of organic matter in a form of forest Development of arsenic ores was associated there with beech litter, and incubated in various moisture conditions, serpentinization of dolomitic marbles and formation of Fig. 1 Location of historical Baltic mines (and sampling sites): Złoty See Stok (ZS1, ZS2), Radzimowice (R) and Dziećmorowice (DM) in the Sudety Mts., SW Poland Odra P O L A N D Legnica 51 Jelenia Radzimowice Góra Wałbrzych Foreland of Sudety Mts. Zloty Stok 0 10 20 30 km CZECH REPUBLIC Kaczawa GERMANY Bystrzyca Nysa Kł. J Soils Sediments various calc-silicate skarn-type rocks (Wierchowiec and with aqua regia (USEPA 3051a). The accuracy of analyti- Wojciechowski 2010). The ores in Radzimowice represent cal method was checked by using two certified reference moderate and low temperature mineral associations that de- materials, CNS392 and CRM044, supplied by Sigma- veloped in postmagmatic hydrothermal processes around a Aldrich. Recovery of As was in the range 92–106% in case rhyolite subvolcanic intrusion of the hill Żeleźniak (Mikulski of CNS 392 (with reference value of 6.49 mg/kg) and 95– 1999). Ore mineralization in DM is associated with quartz- 104% in case of CRM044 (with reference at 57.4 mg/kg). barite and barite-calcite veins that represent low temperature To assess the amounts of As easily soluble in soils, com- crystallization, related to Early Permian porphyries that fill monly accepted extractions with 0.01 M CaCl and 1 M dislocations in the gneisses and migmatites of the Sowie Mts NH NO were performed, according to the procedure by 4 3 block (Muszer et al. 2006). Houba et al. (2000) and ISO 19730 ( 2008), respectively. 2.2 Soils 2.3 Forest litter Four large soil samples were collected from the dumps, Beech forest litter (BL) used in the experiment was collected crushed, and sieved on site to determine the percentage in an early spring (end of March 2016) from a 60-year-old of skeleton and earthy soil (< 2 mm) and then brought to beech woodland. It represented a typical mull with a 15–25- laboratory, air-dried, and homogenized prior to the exper- cm-thick layer of partly decomposed and transformed beech iment. Aliquots of the fine earth fraction were analyzed for foliage. The thickness of this layer tended to decline strongly basic soil properties (Table 1) by standard methods (Tan during the year, and there was no a humification layer. The 2005). Soil texture was determined by a hydrometer meth- beech litter was air-dried, crumbled, and sieved to 1 cm. Its od. Organic carbon and carbonate content was determined analysis involved determination of moisture, organic carbon by an analyzer CS-MAT 5500, Strohlein, total N by content (determined by a carbon analyzer CS-MAT 5500, Kjeldahl method, pH (H O) by potentiometry, and amor- Strohlein), pH in water suspension, and dissolved organic car- phous forms of Fe oxides (Feox) in oxalate extraction. bon (DOC) extracted in cold water, according to the method Total As was determined by ICP-AES (Thermo by Gregorich et al. (2003) and determined by a TOC 5000 Scientific, iCAP 7400), after microwave-assisted digestion Shimadzu analyzer. Table 1 Soil origin and basic soil properties Parameter Unit Soil Symbol ZS 1 ZS 2 R DM Origin Object – Mine dump Mine dump Mine dump Mine dump Locality – Złoty Stok Złoty Stok Radzimowice Dziećmorowice Mined ores – Au and As Au and As polymetallic polymetallic Skeleton (> 2 mm) % 80 50 60 60 Share in fine soil Sand (0.05–2 mm) % 68 76 73 75 Silt (0.002–0.05 mm) % 29 22 22 23 Clay (< 0.002 mm) % 3 2 5 2 Textural group (USDA) SL LS SL LS C org. g/kg 14.1 16.3 18.0 29.5 N total g/kg 1.05 1.23 1.22 1.81 CaCO % 0.1 1.3 –– pH (H O) – 7.1 7.6 4.8 6.5 Feox g/kg 28.55 23.95 23.70 3.50 Total As (aqua regia) mg/kg 45,500 50,000 12,150 196 As extracted with 1 M NH NO mg/kg 8.047.890.07 0.06 4 3 % of total 0.018 0.016 0.001 0.031 As extracted with 0.01 M CaCl mg/kg 24.3 11.3 0.27 < 0.12 % of total 0.053 0.023 0.002 n.d. Presented data are the mean values of three replicates n.d. not determined J Soils Sediments Table 2 Basic properties and fractional composition of beech forest The degree of BL humification and fractional composition litter of its humic substances were determined according to a mod- ified Tiurin’s method, developed and commonly used in Parameter Unit Mean value Polish soil science (Łabaz and Gałka 2012). The operationally defined procedure is based on a multiple sequential extraction of Total organic carbon g/kg 418 acid- and base-soluble fractions, including the steps of extraction N total g/kg 17.4 with acid (fraction 1a), exhaustive extraction with 0.1 M NaOH DOC g/kg 3.95 (fraction 1), as well as the alternate treatment with NaOH and SUVA of water extract L/ mg ∙m 0.28 H SO supposed to destroy the bonds of organic molecules with 2 4 A4:A6 of water extract – 12.7 mineral compounds (fraction 3). Four fractions of humic sub- pH (H O) – 5.95 stances were distinguished: 1a, 1, 2, and 3 (Table 2). The fraction Fraction 1a (fulvic) – 0.05 M H SO - %of total 14.9 2 4 1a contained Baggressive fulvic acids^. The fractions 1 and 2 soluble C (extraction repeated 4 times) contained the mixtures of fulvic and humic acids, and in each of Fraction 1: 0.1 M NaOH-extractable C %of total 37.9 those fractions, the share of fulvic subfraction was determined (multiple, exhaustive extraction). Here, it was repeated 6 times. based on the treatment of its aliquot with 5 M H SO .Thevalues 2 4 Fraction 2: C strongly bound to mineral %of total 25.5 of humic to fulvic acid ratio (CHA:CFA) were determined based compounds (obtained by the alternate on the fractions 1a, 1, and 2, while a humification index was extraction with 0.1 M H SO and 0.1 2 4 calculated based on the share of non-hydrolyzing carbon in BL. M NaOH), repeated 3 times Fraction 3: non-extractable C %of total 21.6 (residual fraction) 2.4 Incubation experiment CHA:CFA in fraction 1 – 0.98 CHA:CFA in fraction 2 – 1.20 Soils were incubated for 90 days in 1-kg pots at various mois- Humification index % 78 tures: 80% of water holding capacity and 100% (water- logged—at maximum water capacity) with and without addi- Fractionation according to modified method by Tiurin (Łabaz and Gałka 2012) tion of beech forest litter BL. The BL was mixed with soil at the rate 50 g/kg on dry matter basis. After 2, 7, 14, 28, and 2.6 Statistics 90 days of incubation, soil pore water was collected with MacroRhizon suction samplers, made of porous plastic mate- The differences among the treatments and times have been rial PES (Eijkelkamp), and examined on chemical composi- analyzed by two-way ANOVA, followed by post hoc multiple tion and optical properties. The experiment was carried out in comparison of means carried out by a Tukey’stest (P <0.05). three replicates, and the data presented in the graphs illustrate Statistical analysis was performed using a software Statistica, the mean values and confidence intervals at p =0.95. version 10.0 (Statsoft). 2.5 Pore water analysis 3 Results Pore water samples were filtered through a 0.45-μmmem- brane and analyzed promptly after collecting. The concen- 3.1 Soil properties trations of total As were determined by ICP-AES AES (Thermo Scientific, iCAP 7400). Additionally, the concen- All mine dump samples were strongly skeletic (50–80%). trations of Mn and Fe that get released into solution under Their earthy fraction that was used in the experiment differed reducing conditions were included in the analysis by ICP- considerably in chemical features (Table 1), correspondingly AES at the end of incubation. The values of pH and Eh were to the sites of their collection and geological settings. The determined potentiometrically, and dissolved organic car- bon (DOC) concentrations were measured spectrophoto- properties of soils ZS1 and ZS2 were similar; they had neutral (ZS1) or slightly alkaline (ZS2) pH and contained high con- metrically, using a standard method with Merck 1.14878.0001 test. The properties of DOC were character- centrations of As: 4.55 and 5.00%, respectively. Soil R contained 1.21% As and had acidic reaction (pH 4.8). Soil ized by A4/A6 and SUVA indices based on the light ab- sorbance at 254, 465, and 665 nm (Leenheer and Croué DM was much poorer in As (196 mg/kg) and indicated a slightly acidic reaction, pH 6.5. All soils, in spite of their initial 2003), determined by an UV-VIS spectrophotometer Agilent Cary 60. Possible effects of nitrate and iron ions stage of development, contained considerable amounts of or- ganic C, which was in the 14.1–29.5-g/kg range. Arsenic ex- present in solutions on the values of SUVA (Weishaar tractability, determined with 0.01 M CaCl and 1 M NH NO , et al. 2003) were checked in selected samples by standard 2 4 3 was by two orders larger in ZS soils than in the others; addition and assessed as negligible. J Soils Sediments however, the amounts of extractable As did not differ consid- untreated soils. A strong increase in As concentrations in soil erably between the soils R and DM despite the large differ- pore water, observed during the first 2 weeks of incubation, ences in their total As concentrations. It is worth noticing the was associated with a strong increase in DOC concentrations As extracted with neutral salts from soil R represented a frac- (Fig. 2). Then, the concentrations of DOC in soil pore water tion of the total As content that was orders of magnitude started to decrease in most of experimental treatments, while smaller in comparison with the other soils. Such effect may the concentrations of As in most cases continued to increase be associated with acidic character of this soil and its parent until the 28 days and in some samples—under anoxic condi- rock, but this hypothesis should be checked in a further study. tions—until the end of incubation (90 days) (Fig. 2). The maximum As concentration in pore water, 384 mg/L, was 3.2 The properties of forest litter reported from waterlogged ZS2 soil, at the end of incubation. The concentrations of Fe in the pore water of all soils treat- Forest litter BL contained 418 g/kg of total organic carbon, in ed with BL and incubated at 80% of water capacity were at the which the share of cold water-soluble organic compounds end of incubation extremely low, i.e., below a determination (DOC) was relatively high, i.e., 3.95 g/kg (Table 2). The data level of ICP-AES (0.1 mg/L). Mn concentrations remained provided by an operationally defined sequential extraction of also low in the pore water of ZS soils; however, a considerable organic matter indicated a pretty large share of acid-soluble, release of Mn from the R soil, and also from DM, was con- Baggressive^ fulvic, fraction 1a (14.9% of total organic car- firmed by increased Mn concentrations in soil pore water bon.), and at the same time a moderate ratio of humic to fulvic (Table 3). A relatively high Eh value (236 mV) indicates that acids (CHA:CFA). The calculated value of a humification this effect was associated with acidic soil pH rather than with index, based on that rate, was considerably high (78%), de- reductive dissolution of Mn. In fact, reductive dissolution of spite the fact that the BL layer in fact did not contain a real Mn in waterlogged soils is governed by the couple of pH-Eh humus, i.e., a well-humified material. It should be stressed that values. Therefore, the concentrations of Mn and Fe in the pore the fraction denoted as the fulvic acids are of a particular water of waterlogged ZS soil samples were at the end of in- importance from the standpoint of possible leaching, as it cubation very low, despite the low values of Eh that are usu- can be dissolved independently on soil pH, whereas the other ally indicative of development of reducing conditions. On the fractions can be solubilized only in alkaline conditions or are contrary, Fe and Mn concentrations in waterlogged R and DM almost insoluble. soils treated with BL were slightly higher or even very high. In the case of R soil, they reached the values of 220 and 310 mg/ 3.3 Initial release of As into solution L, respectively. Spectroscopic analysis of soil pore water from BL-treated soils indicated that generally the values of SUVA The first series of pore water analysis (after 2 days of incuba- started to tion) showed that particularly large amounts of As were re- considerably increase after 14 or 28 days of incubation, and at leased from the soils ZS1 and ZS2. The concentrations of As the same time, the values of A4:A6 started to decrease (Fig. 3). in soil pore water of those soils, in various treatments, were at Such a tendency was observed for all BL-treated soils. This that time in the range 7.4–24.5 mg/L, i.e., by two orders higher simple effect may be considered as a result of a step-wise than those in pore water of R and DM soils, which remained transformation of dissolved organic matter. The dissolved then (after a 2-day incubation) in the range 0.025–0.374 mg/L OM at the end of incubation showed an increased humic char- (Fig. 2). acter, which should probably be ascribed to a progressive Addition of forest litter to all soils increased the mobiliza- residual accumulation of more recalcitrant, aromatic com- tion of As from the soil solid phase into solution; however, pounds derived both from BL and from soil native organic after the first 2 days of incubation, this effect was relatively matter. poorly expressed, particularly in ZS soils, though it became much more pronounced, and statistically significant (Table S1 in the Electronic Supplementary Material), after a longer time 4 Discussion of the experiment (Fig. 2). The concentrations of As in the pore water from various soils 3.4 Effects of incubation time were undoubtedly determined by soil properties and the As speciation in soil. The differences between soils ZS1 and ZS2 The concentrations of As in soil solution tended to significant- on one side and R and DM on the other were apparently ly increase during the first 2 weeks—both in BL-treated and in related to soil properties, in particular to total concentrations untreated soils (Fig. 2). In the soil solutions acquired from the of As in soils (extremely high in ZS soils), and also to its soils treated with BL, As concentrations were by 10-fold or potential solubility in soil solid phase, reflected by extractabil- even more higher than those in the solutions collected from ity in 1 M NH NO and 0.01 M CaCl (Table 1). 4 3 2 J Soils Sediments 250 DOC, mg/L As, mg/ L DOC, mg/L As, mg/ L 200 800 2d 350 2d 7d 7d 14d 14d 150 600 28d 28d 90d 90d 100 400 0 0 0 (80%) BL (80%) 0 (100%) BL (100%) 0 (80%) BL (80%) 0 (100%) BL (100%) ZS 1 ZS 2 1,2 As, mg/ L DOC, mg/L As, mg/L DOC, mg/L 4,0 2d 1,0 2d 7d 7d 14d 0,8 3,0 14d 28d 28d 90d 0,6 90d 2,0 0,4 1,0 0,2 0,0 0 0,0 0 0 (80%) BL (80%) 0 (100%) BL (100%) 0 (80%) BL (80%) 0 (100%) BL (100%) R DM Fig. 2 Concentrations of As and DOC in soil pore water collected from soils ZS1, ZS2, R, and DM, treated with beech forest litter (BL) and non-treated (0), incubated for 2, 7, 14, 28, and 90 days at the moisture of 80 and 100% of water capacity The enhanced release of As from soils, caused their by oxidized conditions, it is usually attributed to the competi- treatment with BL, was unambiguously confirmed for all tion for sorption sites on Fe oxides, whereas in anoxic the soils included in the experiment, both under the condi- environment, several other processes are involved, includ- tions of 80% moisture and in waterlogged soils. In the ing reductive dissolution of oxides (Bauer and Blodau latter case, the amounts of As released into solutions from 2006; Wang and Mulligan 2006; Moreno-Jiménez et al. all soils were particularly high. This observation stays in 2013; Xie et al. 2015). agreement with a common knowledge on As biogeochem- Two aspects of our study seem to be particularly interest- istry in the soil environment (Kabata-Pendias 2011; ing: (i) the comparison of As release from neutral (or alkaline) Wenzel 2013; Komárek et al. 2013) and with the results ZS soils and from more acidic, carbonate-free soils and (ii) the widely reported in the literature. DOM-induced mobiliza- long-term changes in As solubility under oxidized and anoxic tion of As from soils involves various mechanisms. Under conditions. Table 3 Mn and Fe Soil Oxidized conditions, 80% moisture Anoxic conditions, 100% moisture (waterlogged soils) concentrations, as well as pH and Eh values, measured in pore water Fe, mg/L Mn, mg/L pH Eh, mV Fe, mg/L Mn, mg/L pH Eh, mV of BL-treated soils after 90 days of incubation a a a b b b a a ZS1 < 0.100 <0.024 7.42 180 1.194 1.26 7.62 65 a b a b b a a a ZS2 < 0.100 0.459 8.32 135 0.819 0.069 8.30 38 a a a b b b b a R < 0.100 12.5 5.80 236 220 310 6.11 130 a a b b b b a a DM < 0.100 0.122 6.95 210 3.4 9.5 6.70 110 Same letters indicate the mean values of parameters that do not differ significantly (at P < 0.95) between oxidized and anoxic conditions J Soils Sediments Fig. 3 The changes of spectral SUVA parameters A4:A6 and SUVA of pore water collected from BL- treated soils A4:A6 ZS1+BL (80%) 2d 7d 14d 28d 90d ZS1+BL (100%) ZS2+BL (80%) ZS2+BL (100%) R+BL (80%) R+BL (100%) 2d 7d 14d 28d 90d A massive release of As from ZS soils, much more inten- waterlogged, ZS soils remained high for a long time. Apart sive compared to R and DM soils, may partly be explained by from the reductive dissolution of adsorbing soil phases, i.e., the differences of their pH values. Relatively low concentra- oxides (Bauer and Blodau 2006;Blodauetal. 2008; Wenzel tions of As in the pore water of DM soil should obviously be 2013; Shaheen et al. 2014), that takes place under anoxic interpreted in relation to the much lower total As concentra- conditions, also As (V) gets reduced to As (III), a species that tion in that soil (196 mg/kg) compared to the three others (with has much lower affinity to soil sorption sites. Moreover, the over 1% of As). Soil pH plays, obviously, a very important sorption of As (III) is believed to decrease with increasing pH role in governing As solubility, and basically As sorption in in the pH range 8–10 (Casiot et al. 2007). soil, particularly the sorption of As(V), decreases with increas- An initial release of As from all BL-treated soils under ing pH (Wenzel 2013). Various intensity of As release from oxidized conditions (80% moisture), though much smaller soils may also result, however, from different patterns of its than that in waterlogged soils, should be explained mainly speciation in soils and different stability of As-sorbing oxides. by a competition for sorption sites on Fe oxides. A comprehensive study on As mineralogy and recognition of A decrease in As concentrations in soil pore water, in the its species in the soils examined would be necessary to explain latest phases of incubation, except for the waterlogged ZS2, is the differences between two groups of soils. Additionally, the a well-known phenomenon, based on various mechanism, re- intensity of As mobilization from BL-treated ZS soils, partic- ferred to as Baging Bof contaminants (Pigna et al. 2006). ularly high under anoxic conditions, may be attributed to the Similar patterns were described for Cu in forest litter treated, presence of carbonates. Carbonation of arsenic sulfides and contaminated soils (Cuske et al. 2017). This effect may be first formation of stable arseno-carbonate complexes was indicated of all explained by reduction in DOC concentrations, due to by Kim et al. (2000) as an important process in leaching arse- partial mineralization of dissolved organic matter and a selec- nic into groundwater under anaerobic conditions. This mech- tive preservation of the more aromatic, litter-derived mole- anism may also be responsible for the fact that As concentra- cules, which was associated with increasing condensation de- tions in pore water of BL-treated, waterlogged ZS soils, par- gree and aromaticity of particles. Such changes were con- ticularly in ZS2 soil, remained very high during a long-term firmed in case of two ZS soils by a decrease of A4:A6 index incubation (Fig. 2). Small concentrations of Fe and Mn in pore as well as by increase of SUVA after 14 or 28 days of water of those soils, despite their strongly reduced Eh values, incubation. Similar changes in SUVA were described by can be explained by the formation of insoluble carbonates. In Said-Pullicino et al. (2016) in submerged soils. The litter- more acidic DM and R soils, the release of Fe and Mn into derived compounds as well as, most probably, some soil- solution was quite intensive, but the amounts of As released at derived substances released in the long term into the pore the same time were considerably smaller. One more factor water have probably a potential to bind arsenates and to be, should also be mentioned as possible explanation to the fact as such complexes, sorbed onto soil solid phase, contributing that As concentrations in pore water of BL-treated, in that way to As removal from soil solution. A decrease in As J Soils Sediments Bauer M, Blodau C (2006) Mobilization of arsenic by dissolved organic concentrations in soil pore water after longer duration may matter from iron oxides, soils and sediments. Sci Total Environ 354: also be attributed to microbial growth, accompanied by an 179–190 incorporation of the contaminant into microbial biomass. Berg B (2017) Decomposing litter, limit values, humus accumulation, locally and regionally. Appl Soil Ecol. https://doi.org/10.1016/j. apsoil.2017.06.026 Blaser P, Zimmermann S, Luster J, Shotyk W (2000) Critical examination 5 Conclusions of trace element enrichments and depletions in soils: As, Cr, Cu, Ni, Pb, and Zn in Swiss forest soils. Sci Total Environ 249(1):257–280 1. Beech forest litter BL affects strongly the solubility of As Blodau C, Fulda B, Bauer M, Knorr KH (2008) Arsenic speciation and turnover in intact organic soil mesocosms during experimental in mine dump soils and may therefore cause a strong drought and rewetting. Geoch Cosmochim Acta 72(16):3991–4007 increase in As concentrations in soil pore water, first of Bolan N, Mahimairaja S, Kunhikrishnan A, Choppala G (2013) all in waterlogged soils, but also in oxidized conditions. Phosphorus–arsenic interactions in variable-charge soils in relation This effect may cause a release of As into natural water, to arsenic mobility and bioavailability. Sci Total Environ 463:1154– thus being a factor of increasing environmental risk. Brun CB, Åström ME, Peltola P, Johansson MB (2008) Trends in major 2. An increase in As concentrations in soil pore water, in- and trace elements in decomposing needle litters during a long-term duced by forest litter, is a temporary phenomenon that experiment in Swedish forests. Plant Soil 306(1–2):199–210 lasts several weeks and is usually followed by consider- Carbonell-Barrachina A, Jugsujinda A, Delaune RD, Patrick WH, Burlo able decrease along with decreasing DOC concentrations. F, Sirisukhodom S, Anurakpongsatorn P (1999) The influence of redox chemistry and pH on chemically active forms of arsenic in 3. The extent of As release from soil solid phase depends on its sewage sludge amended soil. Environ Int 25(5):613–618 total concentrations in soils and on the composition, miner- Casiot C, Ujevic M, Munoz M, Seidel JL, Elbaz-Poulichet F (2007) alogy, and other properties, in particular pH, of soil parent Antimony and arsenic mobility in a creek draining an antimony material. Soil carbonates seem therefore to be of particular mine abandoned 85 years ago (upper Orb basin, France). Appl importance, as alkaline pH favors As desorption and disso- Geochem 22(4):788–798 Chen Y, Senesi N, Schnitzer M (1977) Information provided on humic lution of organic matter. Additionally, hypothetical forma- substances by E4/E6 ratios. Soil Sci Soc Am J 41(2):352–358 tion of As complexes with carbonates may contribute to As Cuske M, Karczewska A, Gałka B, Matyja K (2017) Would forest litter mobilization under reducing conditions. The release of As cause a risk of increased copper solubility and toxicity in polluted from acidic BL-treated soils likely involved the competition soils remediated via phytostabilization? Pol J Environ St 26(1):419– for sorption sites on Fe oxides in oxidized soils and reduc- Fakhreddine S, Dittmar J, Phipps D, Dadakis J, Fendorf S (2015) tive dissolution of oxides under anoxic conditions. Geochemical triggers of arsenic mobilization during managed aqui- 4. Increasing condensation and aromaticity of dissolved or- fer recharge. Environ Sci Technol 49(13):7802–7809 ganic matter seem to be the considerable factors that con- González AZI, Krachler M, Cheburkin AK, Shotyk W (2006) Spatial distribution of natural enrichments of arsenic, selenium, and urani- tribute to the process of As aging in some of BL-treated um in a minerotrophic peatland, Gola di Lago, Canton Ticino, soils and support its sorption on soil solid phase. This Switzerland. Environ Sci Technol 40:6568–6574 mechanism has not been fully recognized; therefore, the Gregorich EG, Beare MH, Stoklas U, St-Georges P (2003) study on As aging should be continued with considering Biodegradability of soluble organic matter in maize-cropped soils. more comprehensive analytical methods, including speci- Geoderma 113(3):237–252 Han MJ, Hao J, Christodoulatos C, Korfiatis GP, Wan LJ, Meng X (2007) ation of DOM as well as speciation of As both in soil solid Direct evidence of arsenic (III)–carbonate complexes obtained using phase and in the pore water. electrochemical scanning tunneling microscopy. Anal Chem 79(10): 3615–3622 Acknowledgements This research was supported by the National Hindmarsh JT, McLetchle OR, Heffernan LP, Hayne OA, Ellenberger Science Centre of Poland, Project Nos. 2014/13/B/ST10/02978 and HA, McCurdy RF, Thiebaux HJ (1977) Electromyographic abnor- 2016/21/B/ST10/02221. malities in chronic environmental arsenicalism. J Anal Toxicol 1(6): 270–276 Open Access This article is distributed under the terms of the Creative Houba VJG, Temminghoff EJM, Gaikhorst GA, Van Vark W (2000) Soil Commons Attribution 4.0 International License (http:// analysis procedures using 0.01 M calcium chloride as extraction creativecommons.org/licenses/by/4.0/), which permits unrestricted use, reagent. Commun Soil Sci Plant Anal 31(9–10):1299–1396 distribution, and reproduction in any medium, provided you give ISO 19730:2008 (2008) Soil quality—extraction of trace elements from appropriate credit to the original author(s) and the source, provide a link soil using ammonium nitrate solution to the Creative Commons license, and indicate if changes were made. Kabała C, Karczewska A, Medyńska-Juraszek A (2014) Variability and relationships between Pb, Cu, and Zn concentrations in soil solu- tions and forest floor leachates at heavily polluted sites. 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Published: May 30, 2018
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